Raygar Tagton's page

"If you make a suit of armor or weapon out of more than one special material, you get the benefit of only the most prevalent material. However, you can build a double weapon with each head made of a different special material."

ok so prevalent material would eliminate the katana but the RHOKA SWORD consists of two serrated blades placed side by side neither one stated as prevalent so would it be possible?

I know with double weapons you can have one side say Adamantine and the other Mithral. What about items that have characteristics which could possibly support two materials say like RHOKA SWORD states "The sword is the size of a longsword but consists of two serrated blades placed side by side." could one be one metal and the other another metal? or say a KATANA states "katanas employ multiple types of steel combined in a distinctive forging process." since it would be made with Special Materials instead of steel could it be forged with two different metals like the Adamantine and Mithral mentioned earlier? These are just a couple of examples.
This would be nice having benefits of both
Mithral weapons count as silver
Adamantine weapons have a natural ability to bypass hardness.
As for cost you would still have to pay for the special materials of both making it an expensive but possibly beneficial trade off.